Calcium hypochlorite is widely used as a disinfectant and sanitizing agent for supplying available chlorine in the treatment of water supplies such as swimming pool water. To sanitize swimming pool water, available chlorine concentrations ranging from less than 1 part per million to a few parts per million are continually maintained. In conventional methods of application, granular calcium hypochlorite is periodically added directly to the water in the pool in quantities sufficient to maintain the available chlorine at the desired levels. It is preferred, however, to provide substantially continuous application of solid calcium hypochlorite to the pool water.
Placing tablets of calcium hypochlorite in the skimmer or in dissolving baskets around the pool is one method employed. Another method used is to add solid calcium hypochlorite to a dispensing device in which the calcium hypochlorite is contacted with the water to be treated so that the dissolving of the solid is controlled to form a solution of the desired available chlorine concentration. This concentrated solution is then added to the total body of pool water to provide the desired available chlorine concentration.
The operation of these dispensers in treating pool water is substantially trouble-free where the total alkalinity of the water is less than about 100 ppm (expressed as calcium carbonate).
Where the water has a total alkalinity in excess of about 100 ppm (as CaCO.sub.3) and particularly where the pH is high, for example, in excess of 7.8, there is a tendency for the formation and build-up of scale when using dispensers for calcium hypochlorite. Scale build-up is deleterious in that it can block or plug up drains and outlets in the dispenser so o that solution flow is deterred or stopped. This results in inconsistent control of the available chlorine concentration in the water body. Removal of this scale requires, for example, disassembly of the dispensing device and manual cleaning with strong mineral acid. This process is both time consuming and potentially hazardous.
Additives may be introduced to limit the formation of scale, for example, additives which reduce the pH can be added to the pool water or to the dispenser. The use of chemical additives of the scale inhibitor or dispersant types have been shown to control scale in numerous industrial applications. Direct addition of these additives to the pool water, while possible, is not preferred, for example, as a much higher level of additive is required and maintenance of the proper additive level by direct addition to the dispenser is much more difficult to achieve.
Ideally it is desired to have a calcium hypochlorite composition which includes the additive to prevent or inhibit scale formation. In practice, however, it is difficult to find additives which can be directly admixed with calcium hypochlorite. Calcium hypochlorite is a highly active inorganic oxidizing agent which can react readily with oxidizable substances, such as organic substances, in an oxidation-reduction reaction. The addition of chemical compounds, particularly organic compounds, to solid calcium hypochlorite is therefore generally not practiced as the high reactivity of the resultant mixture can result in the release of toxic gases, fire or explosion.
H. Geffers et al in German Patent 2,141,984, issued Aug. 12, 1976 teaches that the use of phosphonopolycarboxylic acids or their salts in active chlorine containing solutions provides good water-softening action. Phosphonosuccinic acid or its sodium salt was added to alkaline chlorine bleach solutions in amounts of at least 5% by weight. Geffers et al also prepared a mixture of 90% of the Na salt of 2-phosphonobutane-1,2,4-tricarboxylic acid and 10% sodium dichloroisocyanurate which was added to water used in bottle washing machines.
The high degree of reactivity of solid calcium hypochlorite, however, is not encountered in aqueous solutions of hypochlorite compounds because of the much lower available chlorine concentrations and the moderating effect of water.
Further, mixtures containing high concentrations of phosphonopolycarboxylic acids or their salts are unsuitable for use in sanitizing and disininfecting water supplies as large amounts of the compositions are required which result in significant cost increases for water treatment.